An acoustic stimulus is represented by the activity it evokes in a population of neurons in the auditory system. Differences between stimuli must be represented by differences between the population of units activated by each stimulus and/or between the activity evoked in the population of relevant neurons. Conversely, any change in the identity of the units comprising the population of activated units and any change in the magnitude of their responses should be associated with a change in the quality of the percept evoked by the stimulus. Thus, both the identity and the activity of the units comprising the populations of neurons activated by various stimuli may underlie the ability to distinguish one stimulus from another. The overall goal of the proposed research is to understand the differential coding of stimuli in terms of both the amount and the spatial distribution, i.e., topography, of activity in auditory cortex. In three studies the collective responses of all isolated single units to a fixed set of monaural and binaural stimuli will serve as an estimate of the topographic organization of activity evoked by those stimuli. In all cases stimulus frequency will be fixed. The resulting maps will be examined within a stimulus set, e.g., 40 dB binaural, within a stimulus dimension, e.g., binaural SPL, and across stimulus dimensions, e.g., contralateral vs. binaural. The temporally dynamic nature of these topographic maps will be examined with a forward masking paradigm in which the interval between the masker and the test stimulus is varied. These studies will be conducted along the extent of an isofrequency strip of AI, over an expanse of AI across isofrequency contours, and in the posterior auditory field. Forward masking can cause a response to a test stimulus that would otherwise not occur or reduce or even preclude a response that otherwise would be robust. These effects would contribute to or determine the dynamic behavior of topographic maps in auditory cortex. Consequently, studies are proposed to investigate the changes in sensitivity of single units in AI and the posterior field in a forward masking paradigm. Qualitative pre-dictions are proposed for EE, EI and TWIN cells. The pilot and expected findings of these studies lead to specific predictions that the perceptual localization of a stimulus is not static. The results of these studies are likely to increase our understanding of the clinical impact of strokes and cortical insult resulting in hearing deficits. Perhaps cortical deafness is related to the instability of its topographic maps.